1. Field of the Invention
The invention relates to a process for the preparation of isoalkenes by catalytic conversion of n-alkenes, a catalyst which is suitable for this conversion, and a process for the preparation of such a catalyst.
2. Discussion of Prior Art
It is generally known that n-paraffins with, for example, 4 to 7 C atoms can be converted to the corresponding isomeric paraffins by using suitable acid catalysts in the temperature range of from 100.degree. to 250.degree. C. Examples of this process are the numerous isomerization processes used in the petrochemical and mineral oil industries for increasing the octane number of light, paraffinic mineral oil fractions. Furthermore, it is known that, in contrast to this, olefins of the same number of carbon atoms cannot be converted to the corresponding isoolefins or can only be converted to the corresponding isoolefins under difficult conditions, for example at very high temperatures, and with poor yield. The attempts hitherto described in the literature for the direct isomerisation of the skeleton of e.g. n-butene to give isobutene or e.g. of n-pentene to give isopentenes at catalysts arranged in a fixed bed are characterized by only initially high yields and selectivities, which diminish and deteriorate considerably after a short period of operation, often after only a few hours. The deterioration in the yields and selectivities is generally attributed to the loss of actively effective catalyst surface or to the loss of active centers. Owing to this high coking rates, formation of oligomers and cracking reactions are observed.
Thus, in U.S. Pat. No. 3,531,542, a process is described for obtaining isobutene from n-butene, in which an Al.sub.2 O.sub.3 catalyst arranged in a fixed bed is employed in a number of stages. In U.S. Pat. No. 3,663,453 the same conversion is conducted also in a fixed bed, with a catalyst consisting of zirconium oxide and an Al.sub.2 O.sub.3 /ZrOCl.sub.2 catalyst. The catalytic isomerization of olefinic hydrocarbons in a fixed bed is also described in U.S. Pat. No. 2,568,964. It is reported that carbon deposits form on the catalyst material during the isomerization process, which reduce the activity and necessitate periodic regeneration of the catalyst. It is stated that the catalyst regains its full activity after regeneration, but one disadvantage which at least remains is that the isomerization process itself has to be interrupted during the period of regeneration.
The following combination of substances may be mentioned in the place of other alkenes, using butenes as examples:
Distillation fractions which contain n-butene, optionally mixed with isobutene, isobutane and n-butane, are often produced in petrochemical plants or in refineries, as, for example, after the separation of 1,3-butadiene from a C.sub.4 -cut or in the cracking of waxy distillates. The optionally present isobutene in such distillation fractions is reacted, above all, in catalytic reactions, for example with methanol to give methyl-tert.-butyl ether, which is separated off by distillation from the residual C.sub.4 -cut, and can be used for improving the antiknocking properties of gasoline or as a solvent. A further catalytic reaction of isobutene in such mixtures is the oligomerization in presence of acid catalysts to give, for example, C.sub.8 -, C.sub.12 - or C.sub.16 -oligomers of the isobutene, which can likewise be separated off by distillation. In both cases, a mixture of n-butenes, n-butane and isobutane remains as a residual product, which can only be put to a minor use, for example as fuel gas, in most cases.
It is therefore desirable again to prepare a mixture containing isobutene from the n-butene/n-butane/isobutane mixture, which remains as a residual product from the abovementioned reactions, by a catalytic conversion, and this isobutene-containing mixture can again be subjected to reactions which are described above in the form of examples.